1. Field of the Invention
The present invention relates to circuits for controlling the output level of switch-mode inverters/converters and, in particular, to combined PWM/FM regulation of resonant switch-mode circuits utilizing direct non-linear control of the frequency to minimize turn-on losses on the switching elements
2. Discussion of the Prior Art
U.S. Pat. No. 4,535,399 issued Aug. 13, 1985, to Tamas S. Szepesi, and commonly assigned herewith, discloses a switching circuit in which frequency modulation (FM) is combined with pulse width modulation (PWM) to control the flow of power from a source to a resonant load.
According to the teachings of the '399 Szepesi patent, a power switch is employed to drive a resonant load from a DC source, whereby the DC input is converted to an AC drive. The power switch is pulse width modulated to control the power flow. The current flowing in the load is sampled and the sample is coupled to the input of a phase locked loop (PLL). The oscillator in the PLL, which is the signal source in the system, is thus phase locked to the load current. The PLL drives a pulse width modulator which also has an input that responds to a DC control voltage. The pulse width modulator operates a drive circuit which provides a waveform which is used to drive the power switch. This combination produces a drive waveform to the load whereby the power switch is actuated precisely at the load current zero crossing. This means not only that the power switch is turned on at zero current, but also that it is turned on at almost zero voltage. As the pulse width of the drive waveform is varied, the frequency of the PLL output is automatically driven to compensate so that zero current turn-on of the power switch is maintained for all pulse widths.
Thus, the circuit described in the Szepesi patent changes the operating frequency automatically so that the changing phase of the driving circuit's describing function due to pulse width modulation is always compensated by an opposite sign change of the phase shift of the resonant load.
Prior to Szepesi's invention, it was conventional to set the zero crossing condition of the pulse width modulator for the maximum power state. For any other pulse width condition, switching did not occur at the zero crossing and switching efficiency was impaired. Thus, the above-described Szepesi circuit represents a significant improvement over the prior art.
While Szepesi's above-described control technique achieves output level control with minimum frequency shift at every duty cycle, and accordingly, can be considered an ideal PWM/FM modulation scheme, it suffers from the disadvantage that its implementation requires relatively complex, expensive circuitry.